As seawater sloshes in and out of inlets under the influence of the moon's gravitational pull, currents are created. These water currents contain substantial amounts of kinetic energy that can be harvested at predictable times to provide power, usually in the form of electricity. River flows are also potential sources of “tidal” power.
Tidal or riverine currents are often low-density power sources, flowing at 3-5 knots or less. This makes the power difficult to extract efficiently. Hydroelectric dams and their coastal cousins the barrage plants amplify the energy density of the water and then harvest the energy using high-pressure, high-velocity turbines. In the process they modify the environment they utilize, often displacing or destroying native ecosystems.
Free-stream turbines have been proposed or deployed that are designed using modern windmill concepts. These “water-windmills” are installed in high-current areas to take advantage of the higher energy density. High current areas are typically “choke points” with lots of marine/riverine mammal, fish, and boat traffic. The choke points are also, in the case of tidal power, highly variable, with reversing currents running from zero to 15 knots or more. It is very difficult to design a turbine to be efficient or even effective in such a variable environment.
In many areas water flows carry debris such as fishing nets and fishing lines, kelp fronds or stalks (which can be extremely tough and entangling), boat anchors and anchor lines, and deadheads and other floating wood. This debris can not only clog or jam the mechanism of a water-windmill, but can build up on the structure or anchoring system significantly increasing the load on the foundation and requiring complicated intervention using divers or underwater vehicles to correct. It is even conceivable that a large water-windmill could snag a small boat's anchor line or fishing nets and pull it under the water.
Water-windmills are usually designed with high Reynolds number, high-efficiency blades. The blades are long and thin, with sharp leading edges. They rotate at fairly high speeds (relative to the marine environment and the animals that live there) with high tip velocities causing damage to either the turbine blade or the struck object in the event of a collision. If the struck object is a small tree (deadhead), only monetary damage is sustained. If the struck object is a whale or a school of migrating salmon, the carnage and subsequent adverse publicity could easily destroy the local tidal power industry.
If a free-stream tidal plant were to cause significant ecological damage, even perceived damage, it would be subject to eco-terrorism and “monkey-wrenching”. Water-windmills would be extremely easy to damage or destroy. Buoy-hung chains launched elsewhere that follow the current into the turbine area will become entangled, requiring difficult servicing or equipment replacement.
Even if a site is chosen that has little or no existing marine life, the very existence of the power plant will create an artificial reef. Various installations (offshore oil rigs, for example) around the world have shown that artificial structure in a “desolate” underwater location soon creates it's own local ecosystem. Filter feeders growing on the foundation and moving parts attract small predatory fish and crabs, eddies and current upsets attract migratory animals, and larger species will soon follow. Power generation installations must be sensitive to all affected marine life, even attracted by the system itself.
A solution is proposed that provides for the extraction of power from flows away from choke points that is not overly susceptible to fouling from debris and that minimizes damage or deleterious effects to the surrounding environment and ecosystem. In addition, the “normal” requirements for low maintenance in an extremely hostile environment and overall cost effectiveness still apply and are addressed.